Fish ladders have long been used to allow fish to migrate past dams and other obstacles in waterways, in order to permit completion of their life cycle and ensure perpetuation of them as species. The most typical and well known device for allowing fish migration has been the use of artificial ponds stepped between a lower, downstream side of a dam and an upper, upstream side, so arranged that water continuously spilled over the tops of the ponds attracts fish and permits them to swim and jump upstream against a current, as fish are wont to do. Ponds of this type, however, must be specially fabricated at each individual cite, due to the unique geography and geometry of each stream/dam combination, and their construction is materials-intensive, so they are expensive to build. Moreover, because the water spilled over the tops of the ponds must run at a constant rate in order to ensure that any fish which are attracted to the ladder are enabled to swim up it, such ladders can be profligate in their release of water which would otherwise remain reserved behind the dam. This leads to loss of undesirably large amounts of water as a valuable stored resource which might otherwise be put to use for drinking, irrigation, power generation, etc. In addition, stepped-pond ladders must typically be installed at the sides of the stream and dam in which they are located, so that insufficient space is available for the passage of fish at peak season, especially on larger rivers or where large migrations occur.
Other solutions have been offered. Among the most promising are those which provide conduits for the fish to use in passing through the stream obstruction, such as the system described in Norwegian patent 179147. However, even such devices are open to improvements of the type described herein.
For example, U.S. Pat. No. 3,596,468 to Fairbanks provides a conduit-type fish ladder. But flow through the conduit is controlled by valves which have single large openings, so that water under full pressure caused by the head between the upper surface of the water way and the valve itself is allowed to pass when the valve is open. This results in a relatively large, high-pressure stream into which the fish are required to swim. The resultant concentrated, turbulent, high pressure stream is often difficult and sometimes impossible for fish to fight, and many of them die in the struggle, or are so tired by the process that they are swept back to the lower end of the conduit and must try again or give up.
Similarly, the valves provided in U.S. Pat. No. 3,232,060 to Windle provide an option of two relatively large holes or a fully opened valve for the fish to combat. In either case very strong, concentrated flows result which are too powerful for fish to fight.